This invention relates to a pad which fits between electronic components and the frame they are mounted on, and in particular to such a pad which is electrically insulating and thermally conductive.
Electronic components, such as integrated circuit chips, are becoming more dense and as a result generate considerable heat. Thus, these components must be mounted on a support frame or a heat sink which dissipates the heat and prevents them from exceeding their operating temperature. Since the frame of an electronic device typically is at ground potential, however, it is necessary to electrically isolate the component from the frame in order to prevent current from flowing between them thereby altering the circuitry in the component. While interface pads which serve this purpose are known in the prior art, materials which have high dielectric strength generally have low thermal conductivity. Accordingly, the prior art pads of this type generally do not have a high enough thermal conductivity to dissipate the heat generated by modern components across the distances necessary to create the required dielectric strength.
While diamonds are known to possess high dielectric strength and high thermal conductivity, a method has not been found to create sufficient area of contact between diamonds, which typically are octahedrons, and the elements they interconnect, and support the diamonds rigidly in this configuration without reducing the overall dielectric strength of the resulting pads or incurring considerable costs.
Collard, U.S. Pat. No. 3,678,995, provides an electronic component mounting pad having good heat transfer characteristics by embedding diamonds in a metallic substrate with the surface of the diamond being flush with the surface of the substrate. A thin film of metal is then deposited on top of the common diamond/substrate surface. The pad of Collard has very low dielectric strength, however, and as a result does not serve the same purpose as the pad of the present invention.
Seal, U.S. Pat. No. 3,721,289 overcomes the problem of creating sufficient surface area by sawing the diamonds into blocks having their opposed faces parallel. Sawing the diamonds is an expensive process, however, and heat sinks generated in this manner have an adverse effect on the overall cost of the electronic device.
Custers, et al., U.S. Pat. No. 3,828,848, combines the teachings of Collard and Seal by embedding diamonds in the substrate of a pad and then sawing the top off of the diamonds to create a side which is parallel with the side of the substrate. While somewhat less expensive than Seal, pads constructed according to the teachings of Custers, et al. are nonetheless very expensive to produce.
The present invention overcomes the shortcomings and limitations of the prior art pads of this type by dispersing diamond powder through a thin film of high dielectric strength material, such as "KAPTON." In a first embodiment of the invention the diamonds do not have to be of uniform size, however, the maximum dimension of the diamond pieces must be less than the thickness of the film. The diamonds must occupy at least 25% of the volume of the pad and preferably occupy up to 60% of its volume.
In a second embodiment of the invention, the diamonds fall within a narrow size range and all have a minimum dimension which slightly exceeds the thickness of the film in which they will be dispersed.
In both embodiments the diamonds are mixed with the high dielectric strength material while it is in a fluid state. The material then is formed into a film and cured with the diamond powder in it. Once the film has cured a thin boundary layer of copper or other metal may be bonded to it, by means such as electroless dipping or vacuum deposition. A thicker cover can then be electroplated onto the boundary layer and a substrate clad to the cover on at least one side of the pad. The resulting pad is joined to the electronic component and the frame by solder or adhesive.
Accordingly, it is a principal object of the present invention to provide a pad for placement between an electronic component and its support frame which has high thermal conductivity and high dielectric strength.
It is a further object of the present invention to provide such a pad which is comprised of a thin film of high dielectric strength material which has diamond powder dispersed through it.
It is a further object of the present invention to provide a filler material which can be dispersed through a high dielectric strength film to increase its heat transfer characteristics.
The foregoing and other objectives, features and advantages of the present invention will be more readily understood upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings.